iMovie HD overlap footage question

I need to overlap various clips over one long clip, iMovie HD is saying that each clip has to be longer than 10 minutes. The problem is that the one long clip is around 20 minutes long and the shorter ones are much less than 10 minutes. Is there any way around this? I've been searching the web all morning looking for a solution to no avail so I decided coming here and asking would be the best move.
Any help would be massively appreciated as I've got to have this movie finished before next Monday, that's the day it's being exhibited and I really need it done! Thanks in advance,
V